Monolithic crystal filters are well known in the art. Such filters provide a bandpass filter transmission characteristic having a much higher quality factor (Q) than is attainable utilizing conventional capacitor and inductor components. In addition, monolithic crystal filters are available in exceedingly smaller structures than attainable in conventional bandpass filter designs.
It will be appreciated that monolithic crystal filters are developed using well-known deposition and etch Integrated Circuit (IC) manufacturing processes. The prior art monolithic crystal filters illustrated in FIGS. 1 and 2, are created by the deposition of shaped electrode structures on opposing sides of a quartz crystal wafer 100. These filters may include opposing pairs of electrodes 101, 103, and 102, 104 as in FIG. 2, or separate input and output electrodes 105 and 106, and a common electrode 107 per FIG. 1. The electrical characteristics of such monolithic crystal filters is generally described in an article by H. F. Tiersten entitled "Analysis of Trapped Energy Resonators Operating in Overtone of Coupled Thickness Shear and Thickness Twist", Journal of the Acoustic Society of America Vol. 59, pp. 879, 1976.
A basic generalization concerning the design and operation of a monolithic crystal filter is that the distance hereinafter referred to as the coupling gap between input and output electrodes is inversely proportional to the filter's bandwidth. Furthermore, the surface area and mass of the electrodes are determinative of the filter's center frequency of operation and inharmonic spurious filter response.
In addition, it is generally known that the surface area of an electrode greatly effects the filter's attenuation characteristics as well as the overall impedance of the monolithic crystal filter system. Unfortunately, the relationship between these differing parameters is hopelessly interrelated. Even minor alterations to one will typically effect some if not all of the important functional characteristics of the filter system design. Accordingly, optimal monolithic crystal filter design is as much an art as it is a science.
A recent trend in the art is to develop monolithic crystal filters that operate at exceedingly high frequencies. Accordingly, there exists a need for monolithic crystal filters capable of Intermediate Frequency (IF) filtering in the Very High Frequency (VHF) range. Such a filter should be designed to provide 5th order overtone operation at IF frequencies greater than 120 MHz. It should also possess a passband of at least 30 KHz, while maintaining a minimum of 50 dB attenuation of signals at least 75 KHz away from the passband center frequency, Fc.
In order to resonate at these high frequencies, the electrodes of the monolithic crystal filter must be designed to operate in the overtone mode of oscillation. This typically requires the selection of electrode and coupling gap sizes which provide the desired frequency of oscillation, while minimizing the magnitude of the filter's inharmonic spurious response. Such a filter will reject I.F. spurious signals in the upper frequency portion of the filter's stopband.
For general overtone operation, it has been established that optimal spurious response designs are dependent upon maintaining a specific ratio between the length (L) and the width (W) of the filter electrodes. For example, an optimal spurious response design for third overtone operation requires that the electrode W/L=1.047. Fifth overtone optimal spurious rejection designs are described by W/L=1.145, where the electrode's length (L) is the side of the electrode parallel to the coupling gap's axis of coupling. Unfortunately, conventional electrodes of sufficient mass, coupling gap width, and surface area to support VHF operation, as well as meet optimal spurious response design parameters, experience relatively high levels of impedance.
Because of radio frequency (RF) problems associated with high filter impedance at VHF frequencies, it is very desirable that monolithic crystal filters operating at these VHF frequencies have low impedence values. In addition, the requirements that these monolithic crystal filters provide impedance matching with the circuitry of the radio receivers in which they are typically employed further points out the need for low impedance filters.
It would therefore be extremely advantageous to provide a wide-bandwidth monolithic crystal filter capable of supporting IF filtering in the VHF range, that possesses both an optimal fifth overtone spurious response design, as well as low impedance.